Because if you accept religious arguments, you can also perfectly argue using similar arguments that also the Flying Spaghetti Monster (FSM) exists.
As long as you don't use religious arguments in an academic or professional context, it doesn't matter what unprovable thing you believe in.
I'm religious, but if we met in an academic or professional context, you'd likely think I'm agnostic because I try to limit myself to arguments that can be proven using observable and repeatable evidence.
If you believe the Flying Spaghetti Monster exists, then have at it. Just don't try to justify a bunch of colanders as a business expense as insurance against an angry FSM, but feel free to wear one as a religious symbol on your own dime.
[edited for clarity - original post was "why is that?"]
I know a number of people, including in my family, that describe themselves as religious one way or another, despite the fact they don't actively participate in any religious practice even down to going to church at all.
We celebrate Christmas and Easter because tradition, and that seems to contribute to this passive religious-ness.
I personally don't care what other people believe in, because it rarely affects how I interact with them on a daily basis. Good luck to them. However, as an atheist I'm not really discomforted with religion and spirituality, it's more I find it an interesting phenomena; the blatant hypocrisy 'they' often display in claiming something is wrong or crazy in comparison to their chosen position (passively or otherwise). It points to a mental processing flaw; an inability to see logic when it threatens their world view. The human condition.
What exactly is “spirituality”?
Dictionary says it’s “related to soul or spirit” - to paraphrase.
But if you don’t believe in souls/spirits ...
Edit: Nice. No retort. Just downvote.